1. Field of the Invention
This invention relates generally to the noninvasive measurement of biological parameters through near-infrared spectroscopy. More particularly, the invention relates to the use of fiber optics for the illumination of analyte samples.
2. Discussion of the Prior Art
Over the past decade or so, near infrared (NIR) spectroscopy has been used in the food and agriculture industries to analyze ground wheat and other samples, See, for example, P. Williams, K. Norris, eds., Near-Infrared Technology in the Agricultural and Food Industries, American Association of Cereal Chemists, St. Paul Minn. (1987) More recently, NIR has found increasing use in pharmaceutical and biomedical application, including the non-destructive monitoring of pharmaceuticals and the transcutaneous measurement of analytes in biological tissue. See C. Horland, B. Davies, Proc. SPIE 1320, 46 (1990) and R. Robinson R. Eaton, R. Haaland, G. Koepp, E. Thomas, B. Stallard, P. Robinson. Clin. Chem, v. 38, 1618-1622 (1992) and J. Burmeister, M. Arnold, G. Small, Diabetes Technology and Therapeutics, v.1, 5-16 (2000) and S. Malin, T. Ruchti, T. Blank, S. Thennadil, S. Monfré, Clin Chem, v. 45, 1651-1658 (1999) or O. Khalil, Clin Chem, v. 45, 165-177 (1999). NIR measurement is performed by directing broadband NIR light through a sample and comparing the spectrum of the incident light to the spectrum of the light that exits the sample. The calculated absorbance spectrum provides a measure of optical density of the sample as a function of NIR wavelength. Individual chemical species have characteristic shaped NIR spectral profiles that typically overlap the spectral features of other species leading to a complex aggregate spectrum that is comprised of the spectral signatures of all NIR active components contained in the sample. The spectral contributions of individual species can be quantitatively evaluated using multivariate mathematics.
Advantages of NIR measurement include nondestructive, noninvasive analysis of the sample, high signal to noise ratios, deep penetration of the sample and the option of using fiber optic technology. Of the disadvantages, the most obvious is poor selectivity due to the characteristically overlapped NIR spectral bands of sample constituents. Highly overlapped spectral bands require the use of multivariate calibration mathemetics and substantial numbers of calibration spectra, with associated glucose values to develop models capable of extracting the relevant analyte information.
To those knowledgeable in the art, the size, arrangement and number of detection and illumination optical fibers at the interface of a probe designed to launch light toward and collect light from a tissue sample, such as human skin, significantly impacts the received signal.
Various attempts have been made in the past to provide devices that illuminate and collect light from a tissue sample. See, for example, K. Maruo, K. Shimizu, M. Oka, Device for Non-invasive determination of glucose concentration in blood, European Patent Application No. EP 0 843 986; and R. Nordstrom, M. Modell, A. Zelenchuk,; and R. Nordstrom, M. Modell, A. Zelenchuk, Systems and methods for optical examination of samples, U.S. Pat. No. 6,411,838 (Jun. 25, 2002).
However, such known devices have provided less than satisfactory results. In particular, the prior art devices have been unsuccessful at compensating for variations in skin thickness at the tissue measurement site. Accordingly, the illuminating beam often over-penetrates the skin, with too much light traveling into the adipose layer. The resulting fat band increases the level of noise and interference in the resulting sample spectrum. It would present a significant technological advance to provide an optical probe that was optimized to target the cutaneous layer of the tissue sample, thereby minimizing interference from the fat band in spectral measurements.